Coal-fired combustion is one of the major sources of energy in the world today; however, it is also a source of major air pollutants such as mercury. With expectations being that the government will impose more stringent regulations on allowable levels of mercury emissions, utilities are actively seeking effective and inexpensive technologies for reducing mercury emissions.
It is commonly accepted that mercury volatizes during coal combustion. Thermodynamic calculations suggest that at combustion temperatures the mercury is present in flue gas in its elemental form (Hg0). As flue gas temperatures cool below 1000° F., mercury is partially converted to the oxidized form (Hg+2). The primary mechanism for mercury oxidation is through reactions with chlorine containing species, resulting in most oxidized mercury being present as HgCl2. Typically, the extent of mercury oxidation in flue gas is greater for coals with higher chlorine content, such as bituminous coals, than for coals with lower chlorine content, such as low-rank coals.
Mercury can be adsorbed by the unburned carbon fly ash by forming a particulate bound mercury. Particulate bound mercury may be removed with other particulate matter by a particulate collection system. While elemental mercury is generally unreactive, oxidized mercury is more reactive than elemental mercury and water soluble, enabling removal of oxidized mercury by equipment such as wet desulfurization systems (wet-scrubbers), typically used to reduce SO2 emissions. The efficiency of most mercury control technologies is therefore dependent on the type of mercury present in flue gas; the greater the oxidized mercury, the greater the efficiency of mercury removal.
The problem of controlling mercury emissions from coal-fired combustion has been addressed by activated carbon injection and mercury oxidation in flue gas by the addition of chlorine or other halogens. This, however, can be costly by requiring expensive additives and activated carbon to improve mercury emission reductions. Accordingly, there is a need for an efficient and economically desirable method for reducing mercury emissions from coal-fired combustion.